GENETICS Meiosis - The Bio Edge - · PDF file · 2014-02-07Proteins are the link between genotypes and phenotypes ... • Asexual Reproduction- ... • Sexual Reproduction/Meiosis

GENETICS Meiosis

Tuesday, March 12, 2013

PREFACE• Genetics- is the study of heredity and hereditary variation.

• Heredity- aka inheritance, is the transmission of traits one generation to another.

• Innate to heredity is the passing of similar traits and the generation of variation.

• There are important and practical applications that come from our understanding of heredity and hereditary variation.

• Your understanding of genetics requires a comprehensive understanding of meiosis, a special case of cell division.


MeiosisI.Main Idea: Parents pass chromosomes to their offspring, these chromosomes contain genes (the unit of heredity) that control traits.


Remember this?

• (Basic Definition) A unit of inheritance that controls a phenotypic character.

• (Better Definition) A nucleotide sequence along a molecule of DNA that codes for a protein.

• (Best Definition) A region of DNA that can be expressed to produce a final functional product that is either a polypeptide or an RNA molecule.

GenesDNA Proteins Traits

OK, What exactly is a gene?


Genes

ProteinsTraits

Genotype Phenotype?

Proteins are the link between genotypes and phenotypesProteins are the link between genotypes and phenotypesProteins are the link between genotypes and phenotypesProteins are the link between genotypes and phenotypes

Review


• The flow of genetic information involves two processes.

• Transcription

• Translation

• Together these two processes represent gene expression.

DNAGlobal Flow of Information

RNA Protein

Review


Inheritance of Genes• Gametes- are the vehicles that carry genes from one

generation to another.

• Gametes are haploid, they carry only 50% a parent’s genes (these genes are carried in 1 complete set of chromosomes).

• Germ cells are diploid cells (they contain 2 copies of every chromosome) that undergo a special type of cell division that produces unique haploid cells...gametes

• Fertilization- the union of gametes brings 1 set of chromosomes from each parent and produces a single unique cell called a zygote that is diploid .

• The zygote undergoes mitosis over and over again producing trillions of somatic cells (all diploid body cells except germ cells and gametes).


Inheritance of Genes• Asexual Reproduction-

• one parent

• no fusion of gametes

• daughter cells get all of parent’s genes

• daughter cells are identical* to each other and parent cell}Binary

Fission

Mitosis

* Unless of course a mutation occurs!


Inheritance of Genes• Sexual Reproduction-

• two parents

• fusion of gametes

• daughter cells get half of parent’s genes

• daughter cells are completely unique (one of kind cells)


MeiosisII.Main Idea: Fertilization and meiosis alternate in life cycles regardless of variations in life cycles.


Closer look at Chromosomes

This is a karyotype, an ordered display of chromosomes. Notice the 23 types (pairs) are numbered and arranged from long to short. The position of the centromere and the colored banded

patterns are also used when arranging the chromosomes.


Closer look at Chromosomes

Sex Chromosomes- the X and Y chromosomes

they are 23rd pair and they determine the sex of

the offspring (XX=girl) (XY=boy)

Autosomes- the first 22 pairs chromosomes

(all except sex chromosomes)

Homologous Chromosomes-

chromosomes with same, length, centromere

position and they carry genes controlling the same

traits


Diploid-symbol 2n-two sets of chromosomes(maternal & paternal sets)-includes almost all cells-humans 2n=46

Haploid-symbol n-one set of chromosomes(maternal & paternal sets)-only sperm and eggs-humans n=23


Describing Chromosomes

2n = 6

Key

Maternal set of chromosomes (n = 3)

Paternal set of chromosomes (n = 3)

Pair of homologouschromosomes(one from each set)

Centromere

Two nonsister chromatids in a homologous pair

Two sister chromatids of one replicated chromosome


Behavior of ChromosomesKey

Haploid (n)Diploid (2n)

Haploid gametes (n = 23)

Ovum (n)

SpermCell (n)

MEIOSIS FERTILIZATION

Ovary Testis Diploidzygote(2n = 46)

Mitosis anddevelopment

Multicellular diploidadults (2n = 46)

Fertilization and meiosis alternate in sexual life cycles, thus maintaining a constant number of chromosomes in each species from generation to generation.

Human Life Cycle:


Review: Spermatogenesis


Review: Oogenesis


Variation of Sexual Life Cycles

Gametes

Diploidmulticellular

organism

Key


nn

2n2nZygote

HaploidDiploid

Mitosis

(a) Animals

nn

n

nn

Haploid multicellularorganism

(gametophyte)

Spores

Diploidmulticellularorganism(sporophyte)

(b) Plants and some algae

nn

n

n

n


(c) Most fungi and some protists

• Alternation of fertilization and meiosis is common in all cycles

• The timing of these two events differs in each cycle

n

2n2n

2nZygoteZygote

MitosisMitosis

MitosisMitosis

MEIOSISMEIOSIS FERTILIZATIONFERTILIZATION

Mitosis

GametesGametes


Animal Life Cycle

Gametes

Diploidmulticellular

organism

Key


nn

2n2nZygote

HaploidDiploid

Mitosis

(a) Animals

• Meiosis occurs only in germ cells

• Germ cells produce gametes

• Gametes are the only haploid cells

• Zygote grows mitotically into a multicellular diploid organism

n


nn

n

nn


(gametophyte)

Spores

Diploidmulticellularorganism(sporophyte)

(b) Plants and some algae

2n2n

Zygote

MitosisMitosis


Mitosis

Gametes

• Exhibit alternation of generations-

• Gametophyte- a multicellular haploid organism/stage

• produces haploid spores

• Sporophyte- a multicellular diploid organism/stage

• produces haploid gametes

Plant/Algae Life Cycle


Fungi/Protists Life Cycles n

n

n

n

n


(c) Most fungi and some protists

2nZygote

MitosisMitosis


Gametes

• Meiosis occurs in zygote

• Meiosis does not result in gametes

• Meiosis produces haploid cells that mitotically produce a multicellular haploid organism

• Mitosis produces gametes


MeiosisIII.Main Idea: Meiosis reduces the number of chromosomes from 2 sets (diploid) to 1 set (haploid).


Mitosis and Meiosis• There are many similarities and important differences between

mitosis and meiosis.

• Look for comparisons as we examine each stage of meiosis in slides that follow.

• In the meantime lets begin our examination of meiosis with this fundamental difference between the two processes.

MeiosisMitosisProduces 2 identical

diploid cellsProduces 4 unique

haploid cells


Meiosis OverviewInterphase

Homologous pair of chromosomes in diploid parent cell

Chromosomesreplicate

Homologous pair of replicated chromosomes

Sisterchromatids

Diploid cell withreplicatedchromosomes

Haploid cells with replicated chromosomes

Sister chromatids separate

Haploid cells with unreplicated chromosomes

Meiosis I

Meiosis II

Homologous chromosomes separate

Haploid cells with replicated chromosomes


Meiosis I

Centrosomes(with centriole pairs)

Sisterchromatids

Chiasmata

Spindle

Tetrad

Nuclearenvelope

Chromatin

Centromere(with kinetochore)

Microtubuleattached tokinetochore

Tetrads line up

Metaphaseplate

Homologouschromosomesseparate

Sister chromatidsremain attached

Pairs of homologouschromosomes split upChromosomes duplicate

Homologous chromosomes (red and blue) pair and exchange segments; 2n = 6 in this example

INTERPHASE MEIOSIS I: Separates homologous chromosomesPROPHASE I METAPHASE I ANAPHASE I


Meiosis IITELOPHASE I AND

CYTOKINESISPROPHASE II METAPHASE II ANAPHASE II TELOPHASE II AND

CYTOKINESIS

MEIOSIS II: Separates sister chromatids

Cleavagefurrow

Sister chromatidsseparate

Haploid daughter cellsforming

During another round of cell division, the sister chromatids finally separate;four haploid daughter cells result, containing single chromosomes

Two haploid cells form; chromosomes are still double


Meiosis: A Closer Look• Interphase G1

• cell grows

• organelles replicate

• carries out destined functions

• Interphase S

• replicates DNA

• replicates centrosomes

• cell grows




• Interphase G2

• cell grows



• prepares for divisions

• condenses chromatin into chromosomes (at the very end of G2 or start of prophase)

• Prophase 1

• chromosomes begin to condense

Meiosis: A Closer Look


• Prophase 1 (beginning)

• homologs pair up lengthwise

• synapsis* occurs, homologs connect at the synaptonemal complex

• crossing over* occurs, genetic exchange between corresponding segments of the chromosomes

• Prophase 1 (middle)

• synaptonemal complex disassembles and homologs are loosely joined


*unique to meiosisTuesday, March 12, 2013

Homologous Chromosomes Pairs (aka “tetrads”)


• Prophase 1 (middle)

• each chromosome has an “x” shaped region called the chiasma. It is the point where crossing over has occurred

• nuclear envelope breaks down

• centrosomes migrate

• spindle fibers form



• Prophase 1 (late)

• microtubules attach to kinetochores and move the homologous pairs toward the middle of the cell

• Metaphase 1

• homologous pairs (tetrads*)are now aligned at the metaphase plate

• both chromatids of each pair are attached to spindles from opposite poles


*unique to meiosisTuesday, March 12, 2013

• Anaphase 1

• proteins holding homologs together break down

• homologs separate and move toward each opposite pole

• cohesion remains between sister chromatids and they move as a unit



• Telophase I & Cytokinesis

• each set has a haploid set of chromosomes but amount of DNA is still that of an normal cell

• one of both sister chromatids contain regions of nonsister chromatid DNA

• Cytokinesis begins before telophase 1 is complete



• Cytokinesis

• animal cells create cleavage furrows and plant cells create cell plates

• no DNA replication between meiosis 1and II

• In some species the chromosomes de-condense and reform a nuclear envelope



• Prophase II

• spindles form once again

• microtubules attach to kinetochores and move the sister chromatids toward the middle of the cell



• Metaphase II

• sister chromatids are now aligned at the metaphase plate

• each chromatid of each pair are genetically unique because of crossing over



• Anaphase II

• proteins holding sister chromatids breakdown

• chromatids separate and move toward opposite poles as individual chromosomes



• Telophase II & Cytokinesis

• chromosomes begin to de-condense

• nuclear envelop reforms

• cytokinesis results in 4 haploid cells

• 4 cells are unique and haploid



Animation of Meiosis


Illustrated Comparison


Summary: Mitosis/Meiosis


Production of Gametes


MeiosisIV.Main Idea: Meiosis creates the genetic variation associated with sexual reproduction.


Evolution & Genetics• Mutations created every allele, and will create every new allele in

the future.

• recall alleles are different genes

• Sexual Reproduction/Meiosis shuffles those alleles to produce unique individuals and variation in gene pools

• Meiosis plays therefor a direct role in evolution by providing the necessary variation for natural selection to work on.

• Also, understanding meiosis provides an understanding of the mechanics behind inheritance of traits (genetics)


Evolution & Genetics• The following slides will explore 3 mechanisms that generate

variation in sexual reproduction.

• independent assortment,

• crossing over and

• random fertilization

• This exploration is important for two reasons:

• First, it illuminates the sources of variation that evolution requires.

• Secondly, understanding meiosis and these same mechanisms and that generate variation will provide the foundation for our understanding of inheritance (genetics).


• In metaphase 1, the homologous pairs (tetrads) align randomly at the cellular equator.

• The paternal and maternal chromosomes are randomly oriented towards one of the poles.

• Notice above 3 homologous pairs (tetrads) can arrange themselves in 4 different ways.


• Thus every gamete has a 50% chance of getting a paternal or maternal chromosome for each and number of chromosome pairs.

How many different gametes are produced if this example of meiosis goes to completion?

8Tuesday, March 12, 2013

• Each daughter cell represents one outcome of all possible combinations of paternal and maternal chromosome combinations.

• Mathematically we can calculate the number of possible combinations by using the following equation:

• 2n where n is the haploid number of the organism.

Humans:

...haploid number is 23, so 223 = ~8.4 million

Bottom Line-...humans can create ~8.4 million different gametes

from independent assortment aloneTuesday, March 12, 2013

Random Fertilization• Although the process of ovulation is not random the actual

oocyte that is ovulated is completely random.

• Although some sperm are more fit than others, the actual sperm that fertilizes the oocyte is completely random.

• This brings us back to the math... (8.4million)(8.4million)=

• ...if any of the 8.4 million different oocytes could be fertilized by any of the 8.4 million different sperm than we could in produce ~ 70 trillion different zygotes!

• You can see that validity in the age old expression “you are one of a kind”

• But even this number does not tell the whole story of our truly unique nature, the fact is the actual number is far greater than 70 trillion.


Crossing Over• Our examination of independent assortment leads us to believe

that we inherit strictly paternal or maternal chromosomes but this is not the case.

• As a consequence of independent assortment paternal and maternal chromosomes exchange segments creating unique chromosomes different than the one we inherited from our parents.


Crossing Over

Recombinant Chromosomes

Chiasma


Crossing Over

Humans have on average 1-3 cross overs per chromosome pair, depending on the size of

chromosome and the position of their centromere.Tuesday, March 12, 2013

When we consider that crossing over takes place 1-3 times on 23 different pairs, multiply that by the variation that independent

assortment and random fertilization create and you have an astronomically large number of possible individual offspring.

In this simple example crossing over has doubled

the genetic variation possible.


Transition to GeneticsV.Main Idea: The principles of meiosis provide the foundation and framework for understanding the inheritance of traits.


Alleles• Alleles are alternate forms of genes.

• Genes are strings of nucleotides that make up DNA.

• DNA wraps around proteins to form chromosomes.

• During reproduction parents donate chromosomes (carrying the alleles) that determine the traits of their offspring.

BUT, here is the key point! Parents do donate single alleles to their offspring rather single

chromosomes. The chromosomes are packages of hundreds of alleles, to understand inheritance you have to understand the behavior of chromosomes,

in other words meiosis!


The location of an allele/gene on a chromosome is a locus (sing.) or loci (pl.).


We can not see genes so we use symbolic letters to represent the genes we can not see.

symbol for purple flowers

symbol for white flowers

A

a

codes for the same trait, but different “recipe”


A Chromosome Carries Many Alleles/Genes


Follow the chromosome, follow the traits!

4 total gametes but only 2 possible kinds

Law of Segregation-2 alleles in a pair must

separate from each other into separate gametes

inherited from her mother

inherited from her fathersay...mother

(“S”eggs) (“s”eggs)Tuesday, March 12, 2013

We Can Follow Two Traits Simultaneously

8 total gametes but only 4 possible kinds

Law of Independent Assortment-

when a genes for two traits are on different pairs of

homologous chromosomes they assort independently

from each other upon gamete formation.

2 possible alignments


We Can Follow Two Traits Simultaneously

8 total gametes but only 2* possible kinds

Draw the possible alignments in metaphase I.

Law of Independent Assortment-

when a genes for two traits are on different pairs of

homologous chromosomes they assort independently

from each other upon gamete formation.

WHY?


What do all of the last three slides have in common?

We know the possible gametes, the different types of sperm or eggs that “could” be produced by the parent.

1.) We know that offspring result from the fusion of sperm with egg.

2.) If we know the types of possible sperm3.) and we know the types of possible eggs 4.) then we can predict the possible offspring


sperm eggs

FemaleMale


1.) What are the possible sperm?2.) What are the possible eggs?3.) What are the possible fertilizations? 4.) What are the possible offspring?

R

r 2.

r3.

4.

1.

R


sperm eggs

FemaleMale

What if we change it up?

R R R r

rrRRRRRR


R

r

r

R

R


R R


sperm eggs

FemaleMale

What if we change it up again?

R R r

rrRRrrRR

r


R

r

r

R

Rr R r


R R

rrTuesday, March 12, 2013

How many different types of sperm? Eggs?

How many offspring are depicted?

How many different

offspring are represented?

What patterns do see in all

gametes?

What patterns do see in all offspring?


In Summary• Genetics- the study of heredity and hereditary variation.

• Heredity- the transmission of traits one generation to another.

• Trait- one or more detectable variants in a genetic characteristic.

• Characteristic- an observable feature that may vary among individuals.

• Gene- a discrete unit of hereditary information consisting of a specific nucleotide sequence in DNA that is responsible for characteristics.

• Chromosome- a cellular structure carrying genetic information (genes)


Transition to Genetics

It is amazing to think that our knowledge of genetics was born in a garden before we knew about chromosomes, genes and DNA.

BUT, here again is the key point! Chromosomes carry genes, genes control traits and genetics

studies the transmission of these traits.

Understanding chromosomes transmission from one generation to another is an essential piece of

knowledge in the “genetic puzzle”


Documents

GENETICS Meiosis - The Bio Edge - · PDF file · 2014-02-07Proteins are the link between genotypes and phenotypes ... • Asexual Reproduction- ... • Sexual Reproduction/Meiosis